Bit of a late response, but figured it needed answering.

Trying to make your own hodge-podge cable is unlikely to work and you’re liable to damage at minimum the charging circuit of the X batt, if not potentially more. Even if you got a cable that would pass type C safeties and detections, you’re unlikely to be able to do anything useful with it as the internal charging circuit is designed to just take power in and charge the battery until it’s full. You don’t have a good way to meaningfully control what it does. I’d say the best way to achieve “storage charge” would just be drain it then do a part charge.